1. Field of the Invention
This invention relates to a method for determining the distance to an object, and is especially, but not exclusively, applicable to automotive radar systems which have microwave sensors to detect obstacles using a frequency modulated carrier combined with a pulsed transmission.
2. Description of the Prior Art
One of many systems, such as those described in WO 03/044559 A1, WO 03/044560 A1, U.S. Pat. No. 6,646,587 B2, JP 2000275333, JP2004333269 and JP2004144696, employed for automotive warning and collision avoidance is frequency modulated interrupted continuous wave (FMICW) radar. In such a system, shown in a block form in FIG. 1, the frequency of a carrier generated by an oscillator OSC is linearly swept in a periodic fashion with a period TSW over a predetermined frequency range ΔF using a frequency modulator FM in a voltage controlled oscillator VCO. A modulation pattern is provided by a linear waveform generator LWG under the control of a control module CM.
The frequency modulated continuous wave (FM-CW) signal is coupled by a coupler CPL to a power amplifier PA where it is amplified, and then gated by means of a transmit-receive switch TRS triggered by the control module CM and operating at a pulse repetition frequency PRI. The transmit-receive switch TRS periodically couples the output of power amplifier PA to an antenna AN for a short interval ΔTT to obtain a pulsed RF transmission signal TX directed towards an obstacle OB of interest. During this interval, which is usually a small fraction of a gating period TPRI=1/PRI, the switch TRS keeps the radar receiver disconnected from the antenna. The reflected signal RX, delayed by a time τ proportional to the object distance D, is detected by the same antenna and coupled to a low-noise amplifier LNA via the transmit-receive switch TRS.
The pulse signal reflected from the obstacle is mixed in a downconverter DR with a reference signal formed by a version of the transmitted signal received from the coupler CPL. Because the transmitted and received pulsed signals are mutually delayed, the instantaneous frequencies of the transmitted and received pulsed signal, are different. Therefore, the beat signal obtained at the output of downconverter has a differential frequency FD, which is directly proportional to the unknown distance D to the obstacle.
The output of the downconverter DR is delivered to a signal processor module SPM, which comprises an analogue-to-digital converter ADC and a digital processor DP driven by clock pulses from a clock CLK. The converter ADC converts the signal from the downconverter DR into a digital signal used by the digital processor DP to determine the beat frequency and hence object range.
The modulation pattern provided by the linear waveform generator LWG may follow, for example, a periodical triangular waveform with a constant slope, as shown in FIG. 2a. Employing this particular waveform is often preferred to other linear modulation schemes (such as sawtooth) since it also allows estimation of the velocity of moving obstacles from the Doppler frequency calculated from a pair of differential frequency shifts derived from transmitted and received signals at rising and falling parts of the triangular waveform.
FIG. 2b shows pulsed signals observed at various points of the system of FIG. 1. It can be seen that the operation of switch TRS ensures that the reflected signal is coupled to the radar receiver only during predetermined time slots ΔTR, which are outside time slots ΔTT used for sending the signal from the transmitter. Such a gating scheme minimises strong signals originating from antenna coupling, which can lead to unwanted effects in the receiver such as saturation of the receiver amplifier and/or the analogue-to-digital converter ADC.
The relationship between beat frequency and object distance should be preserved over the anticipated range of distances. This requires a high degree of linearity of the frequency sweep of the transmitted signal, which imposes strict requirements on the voltage controlled oscillator VCO in the radar transmitter.
The issues related to the linearisation of frequency sweep in radar have been addressed in various ways. Several methods of improvement have been proposed, either by improving the operation of the radar transmitter to achieve a high degree of linearity of the frequency sweep or by minimising the effects caused by such non-linearities.
An example of a possible solution is provided in U.S. Pat. No. 4,539,565. The hardware implementation of the proposed method, aimed to compensate non-linearities of the frequency sweep introduced in the transmitter, is shown in FIG. 3. The data representing the received signal, which experiences phase rotation due to non-linearity present in a frequency sweep in the transmitted signal, is applied to a linearizer. There, the data samples are shifted in time to coincide with those that would have been present for a linear sweep. Alternatively, the data signal may be mixed with a time-dependent correction signal, the frequency of which accounts for non-linearities in the transmitting signal, to achieve a normalised frequency of the beat signal. The normalized beat signal is applied to a processor to establish its frequency spectrum and to divide it into a plurality of frequency bins, so that a signal falling into the bin indicates a corresponding distance to the ranged obstacle. The proposed method can achieve control of linearity to 0.1-0.5%. This is sufficient for most applications, including FMICW radar used for medium- and long-range collision avoidance, where linearity needs to be maintained over time intervals significantly shorter than duration of frequency sweep TSW.
In FMICW radar, the envelope, and thus the frequency FD of the signal is estimated from the received train of pulses, observed during a single frequency sweep of duration TSW. In order for such estimation to be accurate, it is necessary to ensure that the received signal is observed for a time interval not less than the duration TSW. However, because the period TD=1/FD of the beat signal decreases with measured distance D, for distances shorter than a certain critical value such condition cannot be satisfied. Such a particular case is presented in FIG. 2c. 
Another limitation of short distance performance of FMICW radar results from the above-described gating scheme performed by the switch TRS. As is shown in FIG. 2b, for time-delays τ shorter than duration ΔTT of transmitted pulses, the duration ΔTDR (and hence the energy) of the pulses delivered to the downconverter DR is reduced. The shape of such shortened pulses is more likely to be distorted due to, for example, noise and bandwidth limitations in the amplifier LNA and the downconverter DR. As a result, the sampling process performed in the converter ADC at the rate governed by a clock CLK may not correctly determine the amplitude of the pulse. This may lead to errors in estimating the beat frequency from calculations performed in a digital processor DP, and thus a wrong indication regarding obstacle distance.
From the above it follows that FMICW radar systems exploiting the described signal-processing scheme suffer from performance loss at short distances. It would be therefore desirable to develop a novel signal-processing method and an apparatus for improving the performance of FMICW radar particularly at short ranges in a more efficient way than provided by prior art techniques, especially in applications for collision avoidance or/and warning systems.